Today, various display devices have been under active study and development, in particular those based on electroluminescence (EL) from organic materials.
In the contrast to photoluminescence, i.e. the light emission from an active material as a consequence of optical absorption and relaxation by radiative decay of an excited state, electroluminescence (EL) is a non-thermal generation of light resulting from the application of an electric field to a substrate. In this latter case, excitation is accomplished by recombination of charge carriers of contrary signs (electrons and holes) injected into an organic semiconductor in the presence of an external circuit.
A simple prototype of an organic light-emitting diode (OLED), i.e. a single layer OLED, is typically composed of a thin film of the active organic material which is sandwiched between two electrodes, one of which needs to be semitransparent in order to observe light emission from the organic layer; usually an indium tin oxide (ITO)-coated glass substrate is used as anode.
If an external voltage is applied to the two electrodes, charge carriers, i.e. holes, at the anode and electrons at the cathode are injected to the organic layer beyond a specific threshold voltage depending on the organic material applied. In the presence of an electric field, charge carriers move through the active layer and are non-radiatively discharged when they reach the oppositely charged electrode. However, if a hole and an electron encounter one another while drifting through the organic layer, excited singlet (anti-symmetric) and triplet (symmetric) states, so-called excitons, are formed. Light is thus generated in the organic material from the decay of molecular excited states (or excitons). For every three triplet excitons that are formed by electrical excitation in an OLED, only one anti-symmetric state (singlet) exciton is created.
Many organic materials exhibit fluorescence (i.e. luminescence from a symmetry-allowed process) from singlet excitons: since this process occurs between states of same symmetry it may be very efficient. On the contrary, if the symmetry of an exciton is different from the one of the ground state, then the radiative relaxation of the exciton is disallowed and luminescence will be slow and inefficient. Because the ground state is usually anti-symmetric, decay from a triplet breaks symmetry: the process is thus disallowed and efficiency of EL is very low. Thus the energy contained in the triplet states is mostly wasted.
Luminescence from a symmetry-disallowed process is known as phosphorescence. Characteristically, phosphorescence may persist for up to several seconds after excitation due to the low probability of the transition, in contrast to fluorescence which originates in the rapid decay.
However, only a few organic materials have been identified which show efficient room temperature phosphorescence from triplets.
Successful utilization of phosphorescent materials holds enormous promises for organic electroluminescent devices. For example, an advantage of utilizing phosphorescent materials is that all excitons (formed by combination of holes and electrons in an EL), which are (in part) triplet-based in phosphorescent devices, may participate in energy transfer and luminescence. This can be achieved either via phosphorescence emission itself, or using phosphorescent materials for improving efficiency of the fluorescence process as a phosphorescent host or a dopant in a fluorescent guest, with phosphorescence from a triplet state of the host enabling energy transfer from a triplet state of the host to a singlet state of the guest.
As a means for improving the properties of light-emitting devices, there has been reported a green light-emitting device utilizing the emission from ortho-metalated iridium complex    Non Patent Citation 0001: (Ir(ppy)3:tris-ortho-metalated complex of iridium (III) with 2-phenylpyridine (ppy). Appl. phys. lett., 1999 vol. 75, p. 4. ISSN 0003-6951.
Moreover,    Patent Citation 0001: US 20060008670 (UNIVERSAL DISPLAY CORPORATION). 2006 Jan., 12.
discloses an organic light emitting device having an anode, a cathode and an organic layer disposed between the anode and the cathode, said organic layer comprising a complex having one or more arylimidazole, aryltriazole or aryltetrazole derivative ligands chosen, inter alia, among those complying with formula (A) here below:
wherein m is the number of photoactive ligands, which may be any integer from 1 to the maximum number of ligands that may be attached to the metal (that is to say n can be zero); (X—Y) is an ancillary ligand. Preferred complexes are those having a “tris” configuration (i.e. m=3 and n=0) and wherein the metal is Iridium. Said document specifically discloses the tris-orthometallated complex of formula (B) here below:

Also,    Patent Citation 0002: US 2006024522 (THE UNIVERSITY OF SOUTHERN CALIFORNIA PARK). 2006, Feb. 2.
discloses an organic light emitting device having an anode, a cathode and an organic layer disposed between the anode and the cathode, said organic layer comprising a compound bearing one or more carbene ligands (i.e. a compound having a divalent carbon atom with only six electrons in its valence shell when not coordinated to a metal). Among a wide class of complexes bearing carbine ligands, mention is notably made of those comprising a 1-phenyl-3-methyl-imidazolin-2-ylidene ligand, as sketched in formula (C) here below, wherein the imidazole ring and the phenyl moiety are linked via the nitrogen atom in 1-position of said imidazole ring:

Also,    Patent Citation 0003: EP 1486552 A (SONY CORPORATION). 2004, Dec. 15.discloses heterocycle-containing Ir complexes which emit light in blue to green region, complying with following formula:

Still,    Patent Citation 0004: U.S. Pat. No. 6,687,266 (UNIVERSAL DISPLAY CORPORATION & UNIVERSITY OF SOUTHERN CALIFORNIA). 2004, Feb. 3.
discloses light emitting materials having the structure

wherein both A and B are (hereto)aryl rings. As an example of preferred structure, this document discloses the following compound:

Generally, the light emitting material of the prior art provides electroluminescence emission in a relatively narrow band centered near selected spectral regions, and efforts are devoted to tune these emission bands to make them correspond to one of the three primary colours (red, green and blue) so that they may be used as a coloured layer in an OLED.
Nevertheless, another approach which is currently intensively pursued is to develop white-electroluminescence emission, i.e. emitting in a very broad range of wavelengths covering the whole visible wavelengths domain, so that the emitted light mimes natural white light (e.g. sun light or light emitted from an incandescent lamp).
Earlier white-light LEDs were made from a combination of atoms or molecules with different energy gaps, so that the LED emitted light at many wavelengths, each of them corresponding to one molecule, simulating white light. In practice, however, the different materials used in these devices degrade at different rates, so the spectrum of such white-light LEDs changes over time. This makes them unsuitable for use as lights, which must have a stable spectrum over their entire lifetime.
This spawned one of today's hottest application areas for illuminating homes, offices, and industrial plants.
High-efficiency white OLEDs are also desirable as an alternative to full-color active matrix OLEDs because they can be coupled with color filters to circumvent the problematic shadow mask for RGB (Red/Green/Blue) pixelation in production and can help achieve higher display resolution.
Also white emitting materials can be advantageously employed for manufacturing full-colour displays, e.g. flat panel displays, by fabricating an array of organic light-emitting devices that emits white light on one substrate, and incorporating a colour control or conversion array previously fabricated on the same or another substrate so as to achieve a full-color display.
However, since the foregoing light-emitting materials of the prior art do not display emission in the whole visible region, i.e. their emission bands, generally limited to green, are relatively narrow and centered near selected spectral regions, these materials cannot be successfully employed in application wherein a white emission is required.
It has thus been desired to develop light-emitting materials capable of emitting white light.
Efficient long-lived white-light emitters with good colour coordinates are a recognized current shortfall in the field of organic electroluminescent devices.